The present invention generally relates to a broadband rugate filter and, in particular, relates to one such broadband rugate filter having a sinusoidal index of refraction versus optical thickness profile having a frequency that continuously changes in a single direction.
Rugate filters differ from discrete stacked filters in that the index of refraction of the rugate varies as a function of the thickness of the deposited film. Typically, the optical thickness of the refractive index period determines the reflection band position and the amplitude of the variation of the index of refraction determines the reflection bandwidth. As generally known, multiple reflection bands can be generated by serially depositing individual index of refraction profiles for each reflection band or, alternatively, by superimposing multiple index of refraction profiles and depositing the bands in parallel. The use of superposition allows for increased film complexity without adding to the mechanical thickness of the deposited film. In instances where superimposed indices exceed the material indices or result in excessively high slew rates of the material sources, both serial and parallel techniques can be used.
Conventional rugate filters are designed using pure sine wave gradient index of refraction versus thickness profiles. The bandwidth of the reflection notch is related to the excursion of the index of refraction, i.e., the amplitude of the sine wave. The maximum bandwidth is therefore limited to the maximum difference in the refractive indices of the constituent materials. For typical infrared materials, such as ZnSe and ThF4, this limiting bandwidth is about 15%. In order to make a broader reflection band, reflectors designed for different wavelengths can be deposited serially. The difficulty with this technique is the presence of spectral leaks, or narrow pass bands, due to phase effects between the stacked rugate reflectors.
Consequently, it is highly desirable to provide a broadband rugate filter that substantially completely overcomes the above-recited difficulties of conventional rugate filters.